This program is designed to determine human habitability of a thermal environment as affected by ambient temperature, ambient humidity, clothing, thermal radiation, state of physical fitness, age, sex and acclimation. The program is divided into three areas: #1. Characterization of the Human Thermal Environment: by which a model of heat exchange is used to determine physiological response to various environmental conditions. #2. Physiological Responses to Warmth and Humidity: includes a) development of ways in which thermoregulatory competence is assessed in various age groups and sex and b) assessment of occupational heat strain by knowledge of human heat exchange with the environment. #3. Physiological Responses to the Cold: determines the control of the circulatory and metabolic responses to moderate cold.